The present invention relates to a pulsed machining method for the optimized machining of a contour, the pulsed machining method, in particular, being a laser method, such as what as generally referred to as trepan processing, and the optimization being directed to the attainment of the best possible machining results, such as, in particular, the smoothest possible surfaces.
To an increasing degree, laser beams are being used as a tool for machining materials. The inherent advantage of a laser is, most notably, the lack of direct contact between the laser system and the workpiece, the result being virtually the absence of any kind of wear or abrasion. In addition, when the laser beam is properly focused, very small holes or holes having a very large aspect ratio can be bored.
To machine metallic or ceramic materials, for example, a multiplicity of different types of lasers are available, some of which function in what is generally referred to as continuous-wave operation, and others in what is generally referred to as pulsed operation. The method according to the present invention relates to the pulsed-operation machining method.
Since laser beams have a limited diameter, a hole having a larger diameter than that of the beam can only be produced with the aid thereof when the beam is guided in the manner of a milling cutter along a path, for instance a circular path; thus, when the tool is so to speak set into “rotation” at a circular frequency of fCNC. This allows the quantity of machined material to remain low, so the result is a reasonable increase in the machining times. At the end of the machining process, the unmachined cut-out material can be released from the cut-out contour. This machining process, in conjunction with a pulsed tool, is also referred to as “trepanning.”
In pulsed operation, the laser system emits only short, but very high-energy laser pulses of frequency fL. Due to the locally high energy input, material is vaporized; thus, in the optimal case, no molten material is produced which would have to be expelled and could lead to impurities and to a degradation of the surface quality. However, the case where the material merely becomes molten and must be expelled (for example, due to the high pressure pulse during rapid heating) is also often encountered in the art and, depending on the material to be machined, is unavoidable or is even desired.
The following problem arises during trepanning:
Since, in most cases, the diameter of the tool (pulsed laser beam) is significantly smaller than the diameter of the hole to be bored, respectively the dimensions of the contour to be cut out, the cut-out material can only be completely removed when all of the material ablations produced by the pulses a) adjoin one another and b) extend through the entire material thickness.
This can be accomplished in different ways:
increasing the machining frequency;
very slow, one-time traversal of the contour, so that each individual ablation partially covers the respective last one;
repeated traversal of the desired contour, so that the coverage is achieved following a certain number of “rotations” of the tool.
During each rotation, only one layer of a specific thickness is removed; by repeatedly traversing the contour, thus for a plurality of rotation cycles, as many layers as necessary are removed until the component has been machined through the entire thickness thereof.
In the first case, there are technical limits which must not be exceeded. Moreover, in the process, the advantage of a locally narrowly limited heat input, which is essential in high-precision applications, is lost, in particular, since the workpiece frequently becomes distorted by the one-sided heat input which leads to corresponding deviations in the machining. There are also a number of materials (for example, ceramics) which are only ablatable in response to very high-energy pulses of the kind that continuous-wave lasers are not able to produce.
In the second case, a long machining duration usually follows; in conjunction with the previously mentioned disadvantage of a one-sided heat input, this variant leads to less than satisfactory results.
A procedure in accordance with the third case may absolutely lead to good results since, in this case, the heat input is effectively distributed, so that component distortion plays no or only a minor role. The machining times are dependent on the selection of the parameters (pulse frequency fL of the tool, rotational frequency fCNC of the tool relative to the contour) and are only short enough when a best possible coverage of the individual machining pulses is achieved in the context of a lowest possible number of rotations. It is difficult to set these parameters, in particular the tuning of rotational frequency fCNC and pulse frequency fL to one another, since they are dependent on the particular component and the laser parameters and, under known methods heretofore, for example, necessitate complex calculations or time-consuming trials. Alternatively, one must accept needlessly long machining times and/or suboptimal machining results.